Control of hydrocracking process for constant conversion

ABSTRACT

The level of conversion of a hydrocracking operation is controlled as a function of the amount of carbon determined in the recycle gas stream separated from the effluent of the hydrocracking operation.

United States Patent [191 [111 3,718,577

Nai et al. [451 Feb. 27, 1973 [54] CONTROL OF HYDROCRACKING [56] References Cited PROCESS FOR CONSTANT CONVERSION UNITED STATES PATENTS Inventors: Yuen Chen Nai, Titusvme; Yuan 3,463,613 8/1969 Fenske et al. ..23/23O Yan Tsoung, Trenton, both of NJ 3,592,606 7/1971 Boyd ..23/253 [73] Assignee: Mobil Oil Corporation, New 1 Primary E i -D lbert E, Gantz York, Assistant ExaminerG. E. Schmitkons [22] Filed: July 16 1971 Attorney-Oswald G. Hayes et al.

[21] Appl. No.: 163,161 [57] ABSTRACT The level of conversion of a hydrocracking operation 23/230 231/232 is controlled as a function of the amount of carbon 23/253 R, 2oIS/DIG- 1 determined in the recycle gas stream separated from llltg 27/52 the effluent of the hydrocracking operation. [58] Field of Search ..208/108, DIG. 1; 23/232 E,

23/253 R 7 Claims, 3 Drawing Figures 3L, /4/ COMPRESSOR 20 *7 23 //6 -4 F R 27; /26 6 33 l8 2 f /2 7 t I 22 5 T l REACTOR SEPARATOR f Q N \/0 /5 E A I E g /9 R R 32 i /7 -28 I 2/ A 24 7 COMPARATOR DETECTOR 3/ Figure 5 ELECTROMETER ANODE COLLECTOR 1 FLAME CATHODE JET INPUT OAD RESISTOR POWER SUPPLY /n van/0r; /\/0/ Yuan Chen Tsoung- Yucm 700 By T 1? J Agent CONTROL OF HYDROCRACKING PROCESS FOR CONSTANT CONVERSION BACKGROUND OF THE INVENTION 1. Field of the Invention This invention is directed to a method for determining the conversion level of a hydrocracking operation. More particularly, this invention is directed to the combination of determining the conversion level of a hydrocracking operation and controlling the operation on the basis of that determination to maintain a constant conversion level in the hydrocracking operation.

2. Description of the Prior Art Hydrocracking is' one of the oldest catalytic processes for hydrocarbon conversion. It was originally used in Germany in 1927 to convert lignite into gasoline and later to convert petroleum residues into distillate fractions. However, early hydrocracking applications used extremely high pressures and were very costly to operate. Years of independent research by the petroleum industry were required to develop hydrocracking technology to its present level, and it was not until the 1960s that hydrocracking became a widely used refinery process.

The hydrocracking process consists basically of mixing hydrocarbon feed and hydrogen rich gas at elevated pressures, heating the mixture and contacting the heated mixture with a hydrocracking catalyst in a fixed or fluid catalyst bed housed in one or more reactors in series or parallel. Provisions are made for controlling exothermic temperature rise and for separating and recycling hydrogen-rich gas and unconverted hydrocarbons.

In a typical two-stage hydrocracking process, a highly active and/or selective catalyst used in the second stage may be sensitive to impurities such as sulfur and nitrogen found in the hydrocarbon feed. Thus, the first stage is designed primarily for effecting the removal of these impurities to an acceptable level and effect partial saturation of aromatics while minimizing or restricting significant cracking. The first stage is therefore frequently termed the pretreat stage.

It is known to control the conversion level of a hydrocracking process by monitoring a property or properties of the process such as product A?! gravity or refractive index, and adjusting the reactor in response to temperature in a stepwise fashion through the use of a previously established correlation chart.

SUMMARY OF THE INVENTION In accordance with an aspect of the present invention, there is provided a method of controlling a hydrocracking process conversion level. In the process, a hydrocarbon feed stream is preheated and the preheated hydrocarbon stream is applied to a hydrocracking reactor. The reactor effluent is separated in a high pressure separator into a liquid product stream and a recycle gaseous stream which is composed of hydrogen and light hydrocarbons, mostly C -C hydrocarbons. The method of this invention comprises the steps of maintaining the hydrocarbon feed stream to the hydrocracker at a constant predetermined flow rate, determining the total amount of carbon in the recycle gas stream of the hydrocracking operation, and comparing the total amount of carbon found in the recycle gas stream with a predetermined value indicative of a given conversion level. In response to this comparison, an operating parameter of the hydrocracking operation is adjusted in a direction to change a deviation in the total amount of carbon in the gaseous stream from a desired and predetermined amount of carbon required for a given conversion level. In accordance with another aspect of the invention, there is provided a system for controlling the conversion level of a hydrocracking process wherein a hydrocarbon feed stream is preheated and the preheated hydrocarbon stream is applied to a hydrocracking reactor. A gaseous stream including at least C -C hydrocarbon products produced in the process is separated from a product stream of the reactor. The system of this aspect of the invention comprises means for determining the total amount of carbon in a gaseous stream, and means for comparing the total amount of carbon in the gaseous stream to a predetermined total amount of carbon indicative of a desired conversion level. The system additionally includes means for controlling a parameter of the hydrocracking process in a direction to reduce any deviation of the total amount of carbon in the gaseous stream from the predetermined amount of carbon.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows diagrammatically a hydrocracking process and a method for effecting control thereof by the method of the present invention.

FIG. 2 graphically shows the relationship existing between a hydrocracker C -C product yield and the conversion level of the hydrocracking operation.

FIG. 3 shows a schematic diagram of a circuit for generating a control signal with a flame detector.

DESCRIPTION OF SPECIFIC EMBODIMENTS Referring now to FIG. 1 there is shown a single hydrocracking operation wherein a hydrocarbon feed is introduced to the process by conduit or line 11 connected to a heater 10. Make-up hydrogen rich gas is introduced by conduit 12 for passage through preheat furnace along with hydrogen rich recycle gas in line 13. The combined hydrogen rich gas stream is heated in furnace 10 to an elevated temperature. The heated gas in line 14 is then combined with the preheated fresh hydrocarbon feed in line 15 prior to being charged to a hydrocracking reactor 16. Reactor 16 comprises one or more beds of hydrocracking catalysts through which the reactants pass under conditions of elevated tem perature, pressure and space velocity to achieve a desired conversion. Since the hydrocracking operation is exothermic, it is important to provide for maintaining control on the exothermic temperature encountered. The products of hydrocracking are removed from reactor 16 by conduit 17, and cooled in heat exchanger 18 prior to being passed to separator 19. In separator 19 normally gaseous components are separated from normally liquid components. The gaseous products comprising hydrogen and C C, hydrocarbons are removed from separator 19 by conduit 26 and passed to a compressor 20 before recycle to the process by conduit 13 as shown. The normally liquid products are removed from separator 19 by conduit 21 for passage to a products recovery section including a stripper 22. In stripper 22, C -C hydrocarbons carried along with the normally liquid product are separated and removed by conduit 23. The liquid product which is stabilized in stripper 22 is then passed by conduit 24 to a fractionator tower 25. In tower 25, the liquid products of hydrocracking are separated into products of desired boiling ranges and recovered therefrom by conduits 34, 33, 32 and 31.

FIG. 2 graphically shows a series of curves representative of product distribution and yield as a function of the per pass conversion level of the hydrocracking operation. The abscissa of FIG. 2 represents the conversion per pass of a distribution of products which boil up to about 390F. expressed in weight percent of the total products boiling at or below 390F. converted from the hydrocarbon feed stream passed to the hydrocracking operation. The ordinate of FIG. 2 identifies product distribution and yield of product for different levels of conversion expressed in weight percent. FIG. 2 demonstrates, for example, that the yields of the dry gases comprising C to C hydrocarbons and the C hydrocarbons increase as the conversion level increases and is approximately proportional to the conversion level per pass of the hydrocracking process. Thus, a determination of the amount of C,C hvdrocarbons and/or C, hydrocarbons provides a basis for generating a control parameter of the hydrocracking process which may be relied upon to maintain a predetermined conversion level.

In the process combination represented by FIG. 1, the dry gases, comprising C,, C and C hydrocarbons and most of the C hydrocarbons are recovered in the gas stream removed from separator 19 by conduit 26 along with hydrogen not consumed in the hydrocracking operation.

In accordance with this invention a portion of the gaseous product in conduit 26 is withdrawn by conduit 28 provided with flow control valve 27. For example, the flow of'gas in conduit 26 may he in the order of 5 pounds per hour, whereas the amount withdrawn by conduit 28 is only a small fraction thereof in the order of 0.005 pounds per hour.

The gaseous fraction comprising hydrogen and C to C hydrocarbons in conduit 28 is fed to a device such as a carbon detector which will generate a signal as a function indicative of the total amount of carbon in an aliquot part the split off stream in conduit 28. The signal or function based thereon is indicative of the total amount of carbon in the gaseous stream and this signal is passed to a comparator 35 wherein the total amount of carbon signal is compared to a predetermined total carbon signal indicative of a predetermined desired conversion level. The comparator 35 thereafter generates a differential signal (AC signal) which is indicative of a deviation between the signal representative of the total amount of carbon in the line 28 from the predetermined and desired total carbon signal. In the method of this invention the differential signal (AC) is applied by means represented by line 30 to the heater so as to adjust the amount of preheat applied to the reactant streams in conduits 11 and 12 passed through heater 16. For example, in a particular embodimentthe amount of heat applied to streams 1 l and 12 will be reduced when the AC signal is indicative of a high total amount of carbon. This change and to the extent made of course will be dependent upon the type of product desired from the hydrocracking operation. This is readily apparent from the change in yields between a fraction boiling to 290F. as distinguished from a fraction boiling above 290F. for different conversion levels as shown in FIG. 2. Thus, the amount of heat applied by heater 10 to streams 11 and 12 is increased when the AC signal indicates that the total amount of carbon in the line 28 is below the predetermined amount of carbon since this is indicative of a low conversion level from FIG. 3.

A suitable instrument for generating a signal of the type discussed above is known as a flame detector. The flame detector offers the combined advantages and high sensitivity and a wide range of linearity for both single and dual flame detectors. The primary components of a flame detector are the jet, as shown in the drawing, and collector electrode, also shown in the drawing. In operation a carrier gas and hydrogen-containing stream join at the base of the detector and flow through the jet. The hydrogen is burned at the tip of the jet, producing a plasma of sufficient energy to partially ionize nearly all organic substances. When a sample gaseous stream is caused to be swept through the hydrogen flame, a percentage of the molecules are ionized forming positive and negative ions. The extent of ionization depends on the nature of the compound (molecular structure, degree of unsaturation, etc.) and the temperature of the flame (depending on the ratio of hydrogen/air/carrier gas). By using a line operated DC power supply, such as shown in FIG. 3, a potential is applied across the collector and the jet which attracts the electrons and positive ions to the opposite poles. The jet can be biased so that it is positive or negative with respect to ground. When the jet is biased negative and a compound is ionized under these conditions then the electrons are attracted to the collector and the positive ions are attracted to the jet and its surroundings. The collected electrons will then flow through the biasing circuit, as shown by the arrows on the drawing, and create a voltage drop across the input load resistor. The voltage dropped across the input resistor is then amplified by the electrometer, and the output thereof or signal generated thereby may be presented on a potentiometric recorder. The values of ionization current are quite low and, consequently, very high resistance values must be employed to develop a measurable voltage.

A properly designed flame detector of the type briefly outlined herein is 500 to 1,000 times more sensitive than a good thermal conductivity detector. The flame detector also has a very wide range of linear response. The response of the ionization detector depends on the number of molecules per unit time entering the detector. It is not a direct function of the concentration of these molecules in the carrier gas. This is the opposite to thermal conductivity detector principles. Thermal conductivity detectors, on the other hand, are sensitive to the concentration and respond accordingly. However, since the flame detector response is proportional to the number of molecules per unit time, greater sensitivity is obtained at higher flow rates. Increasing the linear velocity by using a smaller diameter jet also improves sensitivity. Another factor is flame temperature (increased hydrogen flow) which will increase the number of molecules ionized, and thus produce a greater signal current.

The degree of ionization response is roughly proportional to the number of carbon atoms per molecule in any given organic compound. Inorganic compounds such as hydrogen, nitrogen, carbon dioxide and water are not ionized, and therefore, are not detectable.

For the above reasons, the flame detector is considered sensitive to all organic compounds and insensitive to inorganic compounds. A general rule is therefore that a compound must have either carbon-to-carbon or carbon-to-hydrogen linkages to be detectable.

Having thus provided a general discussion of the method and concept of this invention and presented a specific example in support thereof, it is to be understood that no undue restrictions are to be imposed thereby except as provided by the following claims.

We claim:

1. A method for controlling the level of conversion of a hydrocracking operation which comprises,

passing a hydrocarbon reactant and hydrogen rich gaseous material at a predetermined temperature and pressure in contact with a hydrocracking catalyst,

controlling the temperature and pressure of the contact with the hydrocracking catalyst, separating the product obtained from said contact under conditions to recover a dry gas fraction comprising hydrogen separate from higher boiling products of hydrocracking, determining the amount of carbon in said dry gas stream and using said carbon determination to generate a signal representative of the level of conversion of said hydrocracking operation and adjusting an operating parameter of said hydrocracking in response to said signal to maintain a predetermined level of conversion during said hydrocracking operation.

2. The method of claim 1 wherein the preheat temperature of the reactant streams is adjusted in response to said generated signal to maintain a predetermined level of conversion.

3. The method of claim 1 wherein the signal is generated with a hydrogen flame detector.

4. The method of claim 1 wherein gaseous material comprising C to C hydrocarbons comprising the effluent of the hydrocracking operation are determined for carbon content and the carbon content thus determined is used to control the level of conversion of the hydrocracking operation with the parameters defined by FIG. 2.

5. The method of claim 1 wherein the level of conversion of the hydrocracking operation is controlled as a function of the dry gas yield so as to provide a product boiling in the range of 180 to 290F. in excess of 35 wt. percent yield.

6. The method of claim 1 wherein the conversion level of the hydrocracking operation is controlled as a function of the amount of carbon in the gaseous effluent boiling below C hydrocarbons so as to maintain a level of conversion per pass selected from within the range of 50 to percent.

7. The method of claim 1 wherein the amount of carbon in the 'dry gas effluent of a hydrocracking operation is used to generate a signal indicative of the level of conversion of said hydrocracking operation and changes in the level of conversion of said hydrocracking operation are made in response to said generated signal to achieve a desired level of conversion. 

2. The method of claim 1 wherein the preheat temperature of the reactant streams is adjusted in response to said generated signal to maintain a predetermined level of conversion.
 3. The method of claim 1 wherein the signal is generated with a hydrogen flame detector.
 4. The method of claim 1 wherein gaseous material comprising C1 to C4 hydrocarbons comprising the effluent of the hydrocracking operation are determined for carbon content and the carbon content thus determined is used to control the level of conversion of the hydrocracking operation with the parameters defined by FIG.
 2. 5. The method of claim 1 wherein the level of conversion of the hydrocracking operation is controlled as a function of the dry gas yield so as to provide a product boiling in the range of 180* to 290*F. in excess of 35 wt. percent yield.
 6. The method of claim 1 wherein the conversion level of the hydrocracking operation is controlled as a function of the amount of carbon in the gaseous effluent boiling below C4 hydrocarbons so as to maintain a level of conversion per pass selected from within the range of 50 to 100 percent.
 7. The method of claim 1 wherein the amount of carbon in the dry gas effluent of a hydrocracking operation is used to generate a signal indicative of the level of conversion of said hydrocracking operation and changes in the level of conversion of said hydrocracking operation are made in response to said generated signal to achieve a desired level of conversion. 